Myth-busting digital X-ray: part one

on 23 August 2007

There are several misconceptions out there that not only damage the reputation of digital X-ray but can also have an impact on the treatment you offer your patients. I hope that by the end of this series of articles you will understand more about digital X-ray and that if you are going to buy a system or replace an existing one, you will be in a better position to buy the best that is available to suit your requirements. This first article is designed to give you an overview of some of the common digital X-ray myths.

Myth 1: You don’t need 20 lpmm as you can not see it

This is a commonly used phrase that makes no sense to me! It might normally be associated with some companies who do not offer 20 lpmm.

What is lpmm? In simple terms, it shows how many lines you can see per millimetre on an image. The higher the number of lines, the better the image. It is true the eye has difficulty seeing the difference between a 10 lpmm and a 20 lpmm X-ray, but when you use the software to compare them it becomes obvious what you are missing with 10 lpmm. On this page are two examples, both taken with phosphors plate systems, one at 12 lpmm and the other at 20 lpmm, both using the same dose.

So why do you need 20 lpmm? Without it you could be missing caries and that small endodontic file you have placed. Below is a simple calculation of what resolution you need to see an ISO 6 endo reamer.

Myth 2: You only need 256 grey levels, as that is all you can see and a monitor can view

About five years ago monitors were running at 8-bit and only viewed 256 grey levels; now they are more sophisticated and run at 32-bit and show over four million grey levels. In the near future, with the rate of advancement they could be running at double this.

What does this mean for you? The statement ‘you can only see fewer than 100 grey levels’ is true in theory, but only when talking about at any one time. For example, on all digital X-ray systems it is possible to change the lightness and darkness, the contrast and the Gamma. On some it is also possible to change the starting image so you can view the image in different modes, using filters and logarithms. This means that the system with the highest levels of grey allows you to see the most information, even to the point where you can see through metal crowns to the prep underneath, giving you necessary information for your clinical decisions.

Myth 3: Fully digital OPTs/cephs (direct) produce 20 lpmm images

All fully digital OPTs and Cephs produce only between 5 and 6 lpmm. This is due to file size, in some cases, and the fact the others use smaller sensors with prisms to take a large image down to a small sensor.

If an image was produced at 20 lpmm, a standard PC would take about five minutes to open it, as it would be around 20-30 megabytes. So, in real terms it’s the quality of the OPT that counts when buying a digital OPT, similar to a film OPT. If you buy a low budget OPT your images could be affected by the poor OPT quality. In future articles I will explain what to look for when buying an OPT.

Myth 4: Digital X-ray gives you an 80% reduction in dose

Be careful with statistics like this one. It is normally comparing the digital system to D speed film using an AC X-ray unit.

What dose reduction you get depends on what you are using at the moment and which system you buy. One of the new direct systems offers you no dose reduction at all, while some will reduce dose down to exposure times of 0.06 of a second for anteriors and 0.16 for posteriors. But this depends on your X-ray unit, as some are able to reduce both the seconds of exposure and the MAs, giving you a further reduction in dose in real dose to the patient.

Myth 5: You can not print X-rays using a normal printer

Modern printers can now print using photo mode in full colour. By doing this, you produce a greater number of grey levels and thus a better image.

Can you print X-ray? Yes you can, without problem. They are accepted by all dental boards as long as you print on at least high white ink jet paper (remember to tell the printer what you are using).

yth 6: Your choice of monitor does not matter

For digital X-rays the monitor and its resolution are paramount. The higher the resolution, the more information you will see. For example, if your monitor’s default setting is 1280 x 1024 then you need to use it at this setting; if not then the image that you are viewing is already distorted and fuzzy before it reaches your eyes. If you are using an old computer with a new monitor you might also find the computer is not good enough to produce the required resolution.

This used to be the case and still is in some situations. Most direct sensors range in quality between 15 lpmm to 20 lpmm, whereas the indirect range is between 9 lpmm to 22 lpmm. As phosphors plate technology increases so will the number of lpmm.

Deciding which system is best depends on your needs, the practice’s needs and which is most suitable for YOU, an issue that I will expand upon later.

Myth 8: I need a digital intra-oral unit to go digital

A simple rule of thumb is that if you can use fast film then most digital systems will work with your existing X-ray unit.

Some require a lower dose than others but it is only a matter of checking what dose is required before buying (take a test shot).

Myth 9: It does not matter from whom I buy digital as it’s just plug and play

This is clearly not true. Training and back-up is essential. Digital X-ray is, apart from your eyes, your only diagnostic tool. It takes years of training and on-site experience to learn how to configure, train and troubleshoot digital X-ray products.

If you bought a car that was worth as much as some of the digital systems, you would expect the person from whom you bought it to know how the car works inside out. Check the qualifications of the person selling to you, the experience they have in digital X-ray and the engineering back-up that they offer. Also check their availability; it is no good if they have all the credentials but can only get to you some time next year!

Myth 10: Third-party imaging software is just as good as that from the manufacturer

With all digital X-ray systems you get software as standard that has taken the manufacturer thousands of pounds to produce and perfect. This is necessary for their images to be produced in the correct manner. If you use third-party software, this can affect the image quality (going from 16-bit images down to 8-bit images.) Some systems can lose three-quarters of their image quality if used with third-party software.

If at all possible, it is always best to use the software that has been produced to be used with the software.

A final note

These are just a few of the misconceptions of digital X-ray. I hope it was useful. In further issues I will go into why you would choose one system over another, what the difference is between direct and indirect, and what to look for when buying a digital OPT.